Is that actually the case? Beside the very team that maintains the language, what other teams/products do use Go as their main programming language?
Google has the disadvantage of history: a lot of the primary products were written before Go came out, and a lot of the newer products are spin-outs from existing codebases. It's very difficult to switch the language of a large existing codebase, and particularly difficult to switch the language to Go (because its runtime is incompatible with C++ and the JVM). You pretty much need a clean-sheet project, which occurs when there's a new microservice that talks via RPC, but almost never occurs in a primary consumer-facing product.
Most good devs - and basically all devs who make it to L5+ (Senior SWE) - will end up working in multiple languages, though. There are a bunch of proprietary ones as well, some of which are open-source (like Protobufs and Bazel) and a few I'm not at liberty to discuss. L6+ your soft skills (communication, EQ, management, prioritization) become more important than your technical skills.
I’m curious about the split between where C++ is often found and where Java is used.
Is it more like less latency critical (or compute cost) services are in Java, and things like Search core are C++? Are there legacy reasons for one over the other?
If you were going to spend time gaining experience in Java or C++, with a long term plan of working on something really interesting and high impact there, which would it be?